1. Field of the Invention
The present invention relates to an x-ray CT (Computed Tomography) apparatus and, more particularly, to a technique of precisely measuring a three-dimensional (3D) CT image (3D image) of a human body or the like in a large field of view.
2. Description of the Related Prior Arts
As a conventional x-ray CT apparatus, a cone-beam CT apparatus for measuring a 3D CT image of the object is known. The cone-beam CT apparatus is an apparatus for detecting a projection of the object of an x-ray emitted radially (conically) from an x-ray source by a two-dimensional (2D) detector, and reconstructing a 3D CT image of the object. A general cone-beam CT apparatus, as shown in FIG. 22A, rotates a pair of an x-ray tube 1 and a 2D detector 3 around the object 8 and scans the object with the x-ray to obtain a projection. In this case, the orbit of the x-ray source 1 is a circle orbit indicated by a circle 2200. It is, however, known that data necessary for reconstructing a 3D CT image of the object cannot be completely collected with the circle orbit. It causes distortion, artifact, or the like in a reconstructed image, and a problem such that the image quality of an obtained 3D CT image deteriorates arises.
A necessary condition to collect complete data was first taught by a paper of Tuy, xe2x80x9cAn inversion formula for cone-beam reconstructionxe2x80x9d, J. Appl. Math., Vol. 43, 1983, pp. 546-552 (Literature 1). Necessary and sufficient conditions were proved by a paper of B. D. Smith, xe2x80x9cImage reconstruction from cone-beam projections: Necessary and sufficient conditions and reconstruction methodsxe2x80x9d, IEEE Trans. Med. Imag., Vol. MI-4, 1985, pp. 14 to 25 (Literature 2).
In the following description, the condition disclosed in Literature 1 will be referred to as xe2x80x9cTuy""s conditionxe2x80x9d. The Tuy""s condition is that xe2x80x9cthe necessary and sufficient condition to collect complete data is that each of all the planes crossing an object crosses an orbit of an x-ray source at least one pointxe2x80x9d. An example of an x-ray orbit satisfying the Tuy""s condition is described in a paper of H. Kudo and T. Saito, xe2x80x9cFeasible cone beam scanning methods for exact reconstruction in three-dimensional tomographyxe2x80x9d, J. Opt. Soc. Am. A., Vol. 7, 1990, pp. 2169 to 2183 (Literature 3). In the example of the x-ray orbit described in Literature 3, assuming that the object is a human body, it is difficult to use two circle orbits which perpendicularly cross each other. Preferably, a helical orbit is used.
The inventors of the present invention examined the conventional arts and found the following problems. The scanning method realizing the helical orbit described in Literature 3 can be, as shown in FIG. 21, easily realized by rotating a set of the x-ray source 1 and the detector 3 around the object 8 and, simultaneously, moving the set of the x-ray source 1 and the detector 3 in the direction Z of the rotation axis. However, since the field of view of the detector 3 is also deviated in the direction Z of the rotation axis, a problem such that a shaded common region 601 is limited occurs. In other words, there is a problem such that the 3D region which can be reconstructed is small.
An example of the scanning method for solving the problem that the common region 601 is limited is disclosed in Literature 3. According to the scanning method described in Literature 3, as shown in FIG. 22B, a scan is performed while moving the x-ray source 1 along a helical orbit 2202 on a spherical shell 2201 having a center point O of rotation. By always fixing the center of the view field of the detector 3 at the point O, a large common region is assured. In the following description, the scanning method described in Literature 3 will be referred to as xe2x80x9cspherical-helical scanxe2x80x9d. According to a method of realizing the spherical-helical scan described in Literature 3, as shown in FIG. 22C, the object 8 is rotated around the Z-axis, and the x-ray source 1 is moved on a circular arc 2203 having the point O as a center. In the method described in Literature 3, however, the object 8 has to be rotated. In the case where a human body is assumed as the object 8, a problem such that it is difficult to realize the method occurs. To be specific, due to a centrifugal force accompanying the rotation of the object 8, the object 8 moves during a scan, and a problem such that the picture quality of a 3D CT image deteriorates arises. There is also a problem such that a patient being operated or a patient required to rest in bed cannot be rotated.
An object of the invention is to provide a technique of collecting complete data necessary to reconstruct an image of the object in a stationary state, further, a technique of acquiring a 3D CT image of a high picture quality, a technique of easily enlarging a common region, and an x-ray CT apparatus capable of realizing improved accuracy of diagnosis of a lung cancer or the like.
The objects and novel features of the invention will become apparent from the description of the specification and the appended drawings.
A representative invention disclosed in the application will be briefly described as follows. As shown in FIG. 6, a scanner (rotary plate) on which a scanning system including an x-ray source 1 and a two-dimensional detector (serving as image acquiring means) 3 is mounted is rotated around an object. Simultaneously, the axis Zxe2x80x2 of rotation of the scanner (which coincides with the axis of rotation of the scanning system) is tilted with respect to the body axis direction Z of the object 8. The tilting is performed around the center O of rotation (which coincides with the center of rotation of the scanning system) of the scanner as a center. By carrying out the rotation and tilting simultaneously, a helical scan as shown in FIG. 22B can be realized without rotating the object 8. Since the view field O of the two-dimensional detector 3 is always fixed to the object 8, a common region 601 can be enlarged.
In the configuration of the representative x-ray CT apparatus of the invention, a scanner on which a scanning system is mounted, the scanning system having an x-ray source for generating radial x-rays emitted to an object and image acquiring means provided so as to face the x-ray source and acquiring a projection of the object is rotated around the object by rotating means, a tilt angle formed between a surface of rotation of the scanner and a body axis of the object is changed while rotating the scanner, projections are acquired from a plurality of directions, and a CT image of the object is generated and displayed. The tilting means tilts the surface of rotation of the scanner to change the tilt angle while holding the center of rotation of the scanner almost in a predetermined position. Since the tilt angle is changed so that the upper part of the gantry storing the scanner is moved apart from the head of the object, the upper part of the gantry is moved apart from the field of view of the object and the object does not feel fear. The shortest distance between the rotation center of the scanner and the rotational axis of the tilting means is set to be smaller than spatial resolution of the image acquiring means. Consequently, the spatial resolution of a CT image does not deteriorate.
Further, as functions of the tilting means, various functions exist as follows. The tilt angle is changed by using the straight line almost orthogonal to the axis of center of rotation of the scanner as a central axis. The tilt angle is changed by using the straight line almost orthogonal to the axis of center of rotation of the scanner and the body axis of the object as a central axis. A change amount of the tilt angle with respect to a unit rotation amount of the scanner is held to be constant. A period of rotation of (360xc2x0xc3x97N) (where, N denotes a natural number) of the scanner by the rotating means is synchronized with a period from start until end of a change of the tilt angle. A period of rotation of (360xc2x0xc3x97I+xcex1) (where, I denotes a natural number and a denotes an arbitrary angle in a range of 0xc2x0 less than xcex1 less than 360xc2x0) of the scanner by the rotating means is synchronized with a period from start until end of a change of the tilt angle. A change in the tilt angle during rotation of the scanner is temporarily stopped. Thus, by combining the functions, various helical scans can be realized. An effect such that collection of complete data necessary to reconstruct the object and enlargement of the common region can be easily achieved is produced.
The scanner can be tilted by tilting the axis Zxe2x80x2 of rotation with respect to the object 8 fixed to the floor or by tilting the object 8 with respect to the axis Zxe2x80x2 of rotation in a state where the axis Zxe2x80x2 of rotation is fixed to the floor. For example, the tilt of the scanner can be realized by tilting a bed (supporting means) 7 for supporting the object while holding the center of rotation of the scanner almost in a predetermined position. When the bed (supporting means) 7 is tilted, the tilt angle is changed by tilting means so that the upper part of the gantry storing the scanner moves apart from the head of the object, so that the object does not feel fear.
By combining a single or a plurality of rotations of the scanner, tilt in one direction, tilt in the reciprocating direction, stop of the tilt during an operation, and the like, various helical scans can be realized. By measuring and recording the rotational angle and tilt angle in each of the scans and using the recorded information at the time of reconstruction, the picture quality of a reconstructed image can be improved.
A representative x-ray CT imaging method of the invention includes: a step of rotating a scanner on which a scanning system is mounted around an object, the scanning system having an x-ray source for generating radial x-rays emitted to the object and image acquiring means (3) provided so as to face the x-ray source; a tilting step of tilting a surface of rotation of the scanner to change a tilt angle formed between the surface of rotation of the scanner and a body axis of the object; an image acquiring step of acquiring projections of the object; an image reproducing step of generating a CT image of the object from the projections acquired from a plurality of directions while rotating the scanner and changing the tilt angle; and a display step of displaying the CT image of the object. A period of rotation of (360xc2x0xc3x97I+xcex1) (where, I denotes a natural number and xcex1 denotes an arbitrary angle in a range of 0xc2x0xe2x89xa6xcex1 less than 360xc2x0) of the scanner by the rotating means and a period from start until end of a change of the tilt angle are synchronized with each other. The tilting step includes a stopping step in which a change in the tilt angle can be temporarily stopped. The tilt angle can be changed so that an upper part of the gantry housing the scanner is moved away from the head of the object.
Effects obtained by the representative configuration in the invention disclosed in the application will be briefly described as follows. (1) By combining a single or a plurality of rotations of the scanner, tilt in one direction, a change in one direction in the tilt angle formed between the body axis of the object and the surface of rotation of the scanner, a change in the reciprocating directions of positive and negative tilt angles, stop of the tilt during an operation, and the like, various helical scans can be realized. Complete data necessary to reconstruct the object can be collected in a state where the object is stationary. (2) A three-dimensional CT image of a high picture quality can be reconstructed. (3) The common region can be easily enlarged. (4) The improved accuracy of diagnosis of lung cancer and the like can be achieved.
A representative configuration of the invention will be summarized by referring to FIG. 1 as follows. An x-ray CT apparatus includes: a scanner on which a scanning system is mounted, the scanning system having an x-ray source for generating radial x-rays emitted to an object and image acquiring means provided so as to face the x-ray source and acquiring a projection of the object; rotating means for rotating the scanner around the object; and tilting means for changing a tilt angle formed between a surface of rotation of the scanner and a body axis of the object. A three-dimensional CT image of the object is generated from the projections acquired from a plurality of directions while rotating the scanner and changing the tilt angle formed between the surface of rotation of the scanner and the object and displayed. According to the invention, the x-ray CT apparatus can collect complete data necessary to reconstruct an image of the object in a stationary state.
In the following description, a vector of a variable V is indicated by a symbol xe2x80x9cVxe2x86x92xe2x80x9d, and an inner product of vectors Uxe2x86x92 and Qxe2x86x92 is expressed as xe2x80x9cUxe2x86x92xc2x7Qxe2x86x92xe2x80x9d.